TU-C-213CD-09: Dosimetric Impact of the Interplay Effect on RapidArc versus IMRT Delivery of Lung Cancer Treatment Using SBRT

Abstract

Purpose: To evaluate the discrepancy between the delivered and planned doses in volumetric-arc-therapy (RapidArc) for lung SBRT, induced by the interplay effect between tumor, gantry and MLC motion, and to compare the effect to that of plans delivered with IMRT. Methods: For 6 patients receiving lung SBRT treatment and having respiratory-induced tumor motion in the range 0.7 to 1.3 cm superior/inferiorly, 4DCT images and respiratory profiles, measured during CT simulation, were available. The ITV and PTV were contoured. Treatment plans using IMRT and RapidArc (with two arcs) were created using the free-breathing CT (FBCT). Each of the arcs was divided into sub-arcs, such that each single sub-arc was delivered to a specific phase of the breathing cycle. The sub-arcs were sorted and assigned to different phases by temporally associating gantry rotation with respiration. The doses delivered to individual phases were calculated using the sub-arcs and CT images of that phase. Doses were then mapped to the FBCT and accumulated, using Demons-based deformable registration, between the FBCT and respective 4DCT dataset. To account for the interplay effect for IMRT, the dose to the individual phase was calculated by re-computing the initial treatment plan on the dataset of the specific phase and weighting the dose according to delivery time of that phase. Results: ITV minimum and mean doses were reduced by 5.5±7.2% and 0.2±0.9% for RapidArc, and by 4.2±5.8% and 0.5±1.0% for IMRT, respectively, when the interplay effect was accounted for. Decreases in PTV minimum and mean doses were 25.1±27.9% and 2.3±2.4% for RapidArc, and 21.4±23.9% and 2.2±2.7% for IMRT, respectively. Conclusions: The interplay effect reduced the ITV and PTV minimum dose significantly in both RapidArc and IMRT plans. The impact on the ITV/PTV mean doses was minimal. RapidArc dose distributions were somewhat more degraded than those of IMRT.